Excimer lasers and other similar electronic transistion lasers use a lasing species that is an electronically excited, unstable compound that dissociates or is otherwise destroyed kinetically immediately after the lasing transition. Of the excimer lasers, those of the rare gas-halide class are among the most promising because of their high efficiency. Rare gas-monohalide excimer molecules such as KrF, XeCl and XeF, for example, produce ultraviolet (UV) laser radiation resulting from transitions between the B and X states (the letters B and X being conventional references to particular states of the excimer). However, in addition to the UV B.fwdarw.X transition, the XeF rare gas-monohalide alone exhibits laser action on another transition designated as the C.fwdarw.A transition. Unlike the UV B.fwdarw.X transition, the XeF(C.fwdarw.A) laser transition is centered in the visible region of the spectrum at about 485 nm, and exhibits an exceptionally broad spectral width extending to wavelengths as low as 450 nm and as high as 520 nm. Because of its broadband output, the XeF(C.fwdarw.A) laser possesses unique potential for development as a tunable optical source for the blue-green region of the spectrum. In order to generate XeF(C.fwdarw.A) laser oscillation the prior art has used a gaseous mixture comprising a single buffer gas, such as argon for example, together with a small amount of xenon and a donor gas which provides the necessary fluorine atoms.
Electrical excitation in the form of high-energy electron beams or electric discharges has been used to excite the XeF(C.fwdarw.A) laser in the art, but these methods have suffered from severe disadvantages that have limited their potential. In particular, the intense electrical excitation required to produce a sufficient number of excited XeF(C) states results in a very large electron concentration, but the electrons mix together the B and C states of the XeF excimer, thereby providing a channel for the competing UV (B.fwdarw.X) transition that drains away excited states before they can provide the desired visible C.fwdarw.A lasing transition. Also, electrical excitation results in large concentrations of ionized and excited species related to the mixture constituents, several of which absorb within the wavelength region of the C.fwdarw.A transition, thereby limiting the net gain of the laser and severely limiting the buildup of optical flux in the laser cavity. These problems are well recognized in the prior art and are described as fundamental limitations of electrical excitation by W. K. Bischel et al in the Journal of Applied Physics, Volume 52, page 4429, 1981 and in Applied Physics Letters, Volume 34, page 565, 1979. In an effort to avoid the aforementioned problems accompanying use of electrical excitation, Bischel et al used photolytic pumping of mixtures containing XeF.sub.2. This method has proved successful, for improving gain, but the necessary apparatus required adds considerably to the complexity of the system.